Oscillator circuits that have rapid charge/discharge characteristics are a vital part of many system architectures. A sawtooth waveform oscillator is used as a reference signal in the control loops of typical PWM-based power conversion systems, for example. Voltage mode switching DC/DC converter architectures often use this type of waveform to compare against an output voltage error signal, the result of which ultimately determines switch duty-cycle. Most current mode architectures require a sawtooth waveform to use as the basis for generation of a slope compensation signal.
Several oscillator methods are used to generate a non-symmetric waveform such as a sawtooth, but most share a common theme of slowly charging a timing capacitor, then quickly shunting the acquired charge through a low-impedance path. This can, of course, also be done in reverse, by quickly charging the timing capacitor then and slowly discharging it. Such method can be applied both for fast-charge and fast-discharge architectures.
The technique of quickly shunting acquired charge through a low-impedance path is generally adequate when the associated reference of the output waveform is ground, since a simple switched shunt can be employed, as used in sawtooth generator 100 in FIG. 1A. A capacitor C1 is charged by a current source I to produce a gradual rise in capacitor voltage, shown in FIG. 1B. When the voltage on capacitor C1 attains a level indicated by VREF, as determined by comparator COMP, the capacitor is discharged by switch S1 during a time delay td corresponding to the fall time of the waveform. This simple architecture is implemented when the endpoints of the sawtooth waveform are at reference and ground levels, i.e., at VREF and GND, shown in FIG. 1B.
When generation of a similar waveform that has both endpoints suspended between the power supply and ground is desired, however, control of the lower endpoint is not trivial. Finite control propagation delays generally prevent the use of comparative hysteretic techniques for control of the endpoints of a rapidly changing voltage, and undershoot can create large errors in the downslope endpoint, resulting in significant errors in oscillator frequency. Low-impedance clamps commonly are used to eliminate undershoot error at the end of the capacitor discharge period to shunt discharge currents away from the timing capacitor once the lower-reference has been reached, as shown in the example of sawtooth generator 200, FIG. 2A. In the circuit illustrated, the charge applied to capacitor C1 is clamped to ˜VREF2. In order to keep these shunt currents to reasonable levels, however, oscillators that employ lower-reference clamps require increased impedance R1 in the capacitor discharge path through switch S1, and thus have longer discharge times than a ground-referred circuit can produce. Even with reduced discharge currents, these clamp circuits often result in substantial glitching on the associated power supply rail due to transient loading by the shunted discharge current.